The present invention relates to a printing apparatus provided with a sheet conveyor mechanism, designed so that a sheet is held by means of conveyor roller pairs and delivered by means of frictional force, and a skew correcting mechanism for correcting a skew of the sheet fed skewed in the feeding direction.
Printing apparatuses frequently use sheet conveyor mechanisms, in which follower rollers are pressed against driving rollers so that a sheet is held between the driving rollers and the follower rollers as it is fed. The sheet conveyor mechanisms are designed to deliver the sheet by means of frictional force between the sheet and the rollers. The force for the delivery of the sheet is settled depending on the level of the force with which the follower rollers are pressed against the driving rollers.
According to one conventional printing apparatus, however, the type of sheets (standard sheets) to be normally used in the apparatus is supposed, and the level of the force of pressure contact is selected according to the sheet type. If a sheet that is thicker than standard ones is fed, the follower rollers are adapted to yield compressing shock absorbing springs. No special measures are taken to counter the use of sheets that are thinner than the standard ones.
On the other hand, many of the sheet conveyor mechanisms of the printing apparatuses are provided with stopper members for skew correction, which are located on the upper-stream side of a platen so that a skew of a sheet in the feeding direction can be adjusted by running the leading end edge of the sheet against them. This mechanism corrects the skew of the sheet by running the leading end edge of the sheet, skewed with slips between the rollers and the sheet, against the stopper members for skew correction and delivering the other side so that it runs against the stopper members with the delivery the impacted side (preceding side) stopped.
More specifically, the skew of the sheet is corrected by applying a turning moment to the sheet by means of sheet conveyor roller pairs. Primarily, however, a uniform holding force or uniform delivery force is applied to the sheet conveyor roller pairs so that the sheet can be delivered uniformly in the wide direction of the sheet. For skew correction, therefore, the holding force is adjusted so that greater slips are generated between the conveyor roller pairs and the sheet on the side where the sheet abuts against the stopper members for skew correction.
In a conventional sheet conveyor roller pair, however, one driving roller and one follower roller, which extend long in the width direction of a sheet conveyor path, are mounted on a driving shaft and a driven shaft, respectively, and the driving roller and the follower roller are opposed to each other. In consequence, the adjustment of the holding force is uniform in the width direction of the sheet, so that it is hard to obtain an appropriate holding force for adequate slips on one side and adequate feed on the other side. If the individual shafts are mounted so that they are situated at a distance of a value smaller than a set value from each other on one side and at a distance of a value greater than the set value on the other side, the sheet delivery force is greater on the shorter distance side. If the side on which the sheet delivery force is greater is coincident with the side to which adequate slips must be given, therefore, the slips are inadequate, so that there is a possibility of sheet jamming in positions for skew correction or of incomplete skew correction.
In order to correct a skew of a sheet, the sheet is expected to have a resistance high enough to stand the delivery force of the sheet conveyor roller pairs, which produce frictional force as they slide, without bending.
In the case of a sheet thinner than the standard ones or of a high-friction sheet, however, the delivery force of the sheet conveyor roller pairs is inevitably greater than the resistance of the sheet. In many cases, therefore, the sheet is bent or turned up during skew correction, so that the skew correction ends in failure. If the delivery force of the sheet conveyor roller pairs is reduced to fit the skew correction, in contrast with this, it is difficult normally to feed the sheet into a printing gap between the platen and a print head, since the resistance against the passage of the sheet in the printing gap is higher than in any other cases.
Disclosed in Japanese Utility Model Registration No. 2508855, therefore, is a printing apparatus in which soft auxiliary rollers 59 of sponge or the like are mounted individually on the respective outer peripheries of sheet feed rollers 51 (follower rollers) that are pressed against feed rollers 55 (driving rollers), as shown in FIG. 10. In this printing apparatus, a sheet is transported with the sheet delivery force of the sheet conveyor roller pairs reduced by means of the auxiliary rollers before stoppers for skew correction are reached. Thereafter, the sheet feed rollers 51 are pressed strongly against the feed rollers 55 to squeeze the auxiliary rollers 59 so that the sheet can be transported with a great delivery force. In FIG. 10, numeral 52 denotes an openable cover; 53, a shaft; 54, a shank; 56, a shaft; 57, a guide; 59, the auxiliary rollers; and 60, sheet stopper members.
However, this arrangement can only reduce the sheet feeding force before the skew correction and restore the normal feeding force after the skew correction, and cannot be a solution when the type of the sheet and therefore the thickness of the sheet are changed or when the frictional force between the sheet and the sheet conveyor roller pairs (coefficient of friction of the sheet surface) is changed.
Many sheet conveyor mechanisms of printing apparatuses are provided with a skew correcting mechanism, which is located on the upper-stream side of the platen in the sheet feeding direction and is designed so that a skew of a sheet in the feeding direction is corrected by running the leading end edge of the sheet against stopper members of the mechanism. The stopper members are arranged so that they project above a sheet conveyor surface to intercept the sheet conveyor path only during skew correction and are evacuated from the sheet conveyor surface so that they do not hinder the transportation of the sheet in other cases.
According to a conventional skew correcting mechanism (e.g., Japanese Utility Model Registration No. 2508855, mentioned above), however, the stopper members are evacuated with a sheet held against them after the sheet is run against the stopper members to correct the skew. Therefore, the evacuation of the stopper members is incomplete, so that the transportation of the sheet after the skew correction sometimes may be hindered. This is because the sheet held against the stopper members is strongly pressed by means of the feeding force of the sheet conveyor roller pairs for feeding even in that state, the friction between the leading end of the sheet and the stopper members is great, and movable parts of a mechanism for pushing the stopper members in the feeding direction to evacuate the stopper members are subject to pinching. This trouble is liable to occur when a particularly stiff sheet or thick sheet is used.
According to the conventional mechanism, therefore, the evacuation of the stopper members requires a substantial driving force or undue operation, so that the leading end edge of the sheet may be damaged, in some cases.
An object of the present invention is to provide a printing apparatus provided with a sheet conveyor mechanism, whereby skew correction and transportation of sheets into a printing gap can be normally performed even if the thickness and the coefficient of surface friction of the sheets varies depending on the type of the sheets. Another object is to provide a printing apparatus provided with a sheet conveyor mechanism, in which a sheet is given adequate slips on one side in the width direction thereof and adequate feed on the other side during skew correction so that a skew of the sheet can be corrected smoothly. Still another object is to provide a printing apparatus in which stopper members of a skew correcting mechanism can be easily evacuated without damaging the leading end edge of a sheet.
In order to achieve the above object, a first form of a printing apparatus according to the present invention comprises: a body frame; a sheet carrier fixed to the body frame and having a sheet conveyor surface defining a straight sheet conveyor path; a platen exposed to the sheet conveyor path through the sheet conveyor surface of the sheet carrier; a print head opposed to the platen; sheet conveyor rollers arranged on the upper-stream side of the platen in the sheet feeding direction, holding a sheet from both sides, obverse and reverse, and capable of transporting the sheet; a skew correcting mechanism including stopper members arranged between the platen and the conveyor rollers in the feeding direction and capable of projecting above the sheet conveyor path and receding from the sheet conveyor path; and a control device including a sheet sensor. The sheet conveyor rollers are adapted to rotate a set angle in a direction opposite to the sheet feeding direction such that the sheet is separated from the stopper members of the skew correcting mechanism before the stopper members are evacuated from a sheet conveyor path.
In the printing apparatus with this construction, the sheet conveyor roller pairs are reversed to separate the sheet from the stopper members before the stopper members are evacuated from the sheet conveyor path when the sheet correction is completed, so that the evacuation of the stopper members requires no substantial force. Further, there is no possibility of the leading end of the sheet conveyor mechanism being drawn in and damaged as the stopper members are evacuated or of sheet jamming.
Preferably, the stopper members of the skew correcting mechanism project above the sheet conveyor path in synchronism with a sheet detection signal from a paper-in sensor located in the sheet conveyor path on the upper-stream side of the sheet conveyor rollers.
Preferably, the stopper members of the skew correcting mechanism project above or recede from the sheet conveyor path by rotating around a rotating shaft located under the sheet carrier and extending parallel to the sheet feeding direction.
Preferably, the stopper members are in engagement with a driving plate capable of linear motion in a direction to cross the sheet conveyor path under the sheet carrier and rotate as the driving plate makes linear motion.
Preferably, the engagement of the stopper members and the driving plate is engagement between gear teeth of the stopper members equivalent to a pinion of a pinion-rack mechanism and a gear of the driving plate equivalent to a rack.
Preferably, the skew correcting mechanism includes a sub-frame and a solenoid for linear drive fixed to the sub-frame, the sub-frame having the stopper members rotatably supported thereon and the driving plate slidably attached thereto, and the driving plate is coupled with an armature of the solenoid to form one entire unit.
Preferably, the sheet conveyor rollers constitute a conveyor roller pair composed of a first roller and a second roller, a plurality of pairs are arranged in the width direction of the sheet in a manner such that at least the first or second roller is plural, and at least one of the rollers of each pair is independently pressed against the other with a predetermined force.
A second form of a printing apparatus according to the present invention comprises: a body frame; a sheet carrier fixed to the body frame and having a sheet conveyor surface defining a straight sheet conveyor path; a platen exposed to the sheet conveyor path through the sheet conveyor surface of the sheet carrier; a print head opposed to the platen; sheet conveyor roller pairs arranged on the upper-stream side of the platen in the sheet feeding direction, holding a sheet from both sides, obverse and reverse, and capable of transporting the sheet; and stopper members for skew correction arranged between the platen and the conveyor roller pairs in the feeding direction and capable of projecting above the sheet conveyor path and receding from the sheet conveyor path. The sheet conveyor roller pairs include a first roller and a second roller, a plurality of pairs are arranged in the width direction of the sheet in a manner such that at least the first or second roller is plural, and at least one of the rollers of each pair is independently pressed against the other with a predetermined force.
In the printing apparatus with this construction, a plurality of conveyor roller pairs are arranged so that the conveyor roller pairs independently have their respective sheet holding forces, so that the sheet can be smoothly rotated on the sheet conveyor surface during skew correction to correct the skew of the sheet in the feeding direction.
Preferably, the driving roller is supported on the sheet carrier, and the follower roller is pressed against the driving roller with a predetermined force.
Preferably, a plurality of follower rollers are rotatably supported on holders independently swingably supported on the body frame, and urging members for pressing the follower rollers to the driving rollers are arranged between the holders and the body frame.
Preferably, a plurality of the follower rollers are rotatably supported on holders independently swingably supported on the body frame, a movable member capable of position adjustment is arranged between the holders and the body frame, and urging members for pressing the follower rollers to the driving rollers are arranged between the holders and the movable member.
Preferably, the body frame supports a sub-frame for swinging motion such that the follower rollers can be brought into contact with or separated from the driving rollers by swinging the sub-frame, a plurality of follower rollers are rotatably supported on holders independently swingably supported on the sub-frame, and urging members for pressing the follower rollers to the driving rollers are arranged between the holders and the sub-frame.
Preferably, a swing shaft of the sub-frame is provided on the body frame in a manner such that the direction in which the follower rollers are brought into contact with or separated from the driving rollers is a tangential direction of the driving rollers parallel to the sheet feeding direction.
Preferably, the body frame supports a sub-frame for swinging motion such that the follower rollers can be brought into contact with or separated from the driving rollers by swinging the sub-frame, a plurality of the follower rollers are rotatably supported on holders independently swingably supported on the sub-frame, a movable member capable of position adjustment is arranged between the holders and the sub-frame, and urging members for pressing the follower rollers to the driving rollers are arranged between the holders and the movable member.
Preferably, a swing shaft of the sub-frame is provided on the body frame in a manner such that the direction in which the follower rollers are brought into contact with or separated from the driving rollers is a tangential direction of the driving rollers parallel to the sheet feeding direction.
Preferably, the urging members are provided individually for the holders.
Preferably, the force of the conveyor roller pairs to hold the sheet can be adjusted depending on the type of the sheet.
A third form of a printing apparatus according to the present invention comprises: a body frame; a sheet carrier fixed to the body frame and having a sheet conveyor surface defining a straight sheet conveyor path; a platen exposed to the sheet conveyor path through the sheet conveyor surface of the sheet carrier; a print head opposed to the platen; a plurality of sheet conveyor roller pairs arranged on the upper-stream side of the platen in the sheet feeding direction and capable of holding a sheet from both sides, obverse and reverse, with independent forces; a skew correcting mechanism including stopper members arranged between the platen and the conveyor roller pairs in the feeding direction and capable of projecting above the sheet conveyor path and receding from the sheet conveyor path; and a control device including a sheet sensor. The force of the sheet conveyor roller pairs to hold the sheet is adjustable depending on the type of the sheet.